A "Regiment" is a group of players who share a communication channel and are led by a Regiment Commander.
To join a regiment, hit F6 on your keyboard, click on the TOP tab that reads "Regiment," and then find the name of the regiment you wish to join in the top left side of the list. In the columns to the right of the regiment name you should see the regiment commander (top ranked member) and the current captain. If you click on the commander name, you should see the name of the current ranked leader appear in your friends list and you can private message them about joining that regiment.
in the old days you could click on a highlighted regiment and that will allow you to join it
but you had to have the regiments name after yours when in regi chat unlike now with the anything after the start (*)
If you aren't interested in a regiment right away, there's always the option of "mercing".
the regiment codes, the things to now invite/rank/ect are as follows
/regiment (name) - ingame way of making a new regiment, be adviced that it costs 200uranium and unless you have support dont start another regiment please /invite (name) - invite another player into the regiment. Both players must be in the faction capital at time of the invitation. /uninvite (name) - remove a member from the regiment. /changerank (rank#),(rank#) - will move the person in the first specified rank position to the second specified rank position and appropriately sort the regiment ranks. /rank (name) - will change the member from unranked status to the lowest current rank. /unrank (name) - will change the member from a ranked status to unranked status. /rmsg (message) - sets a regiment message for the day viewable when members log on. /rpower - shows regiment powers (PBC/Shout)